1. Technical Field
The present invention relates to a power receiving control device, a power receiving device, and an electronic apparatus.
2. Related Art
In resent years, a contactless power transmission technique, which utilizes electromagnetic induction so as to enable power transmission even without a metallic contact point, has attracted lots of attention. As an application of the contactless power transmission technique, charging of a cell-phone or a household appliance (for example, a cordless handset for a telephone) is proposed.
JP-A-2006-60909, for example, discloses a contactless power transmission device using a primary coil and a secondary coil.
Recently, secondary batteries such as a lithium-ion battery and a lithium polymer battery have been widely used as a battery of a cell-phone or a mobile terminal of a notebook-sized personal computer and the like. The secondary batteries such as the lithium-ion battery and the lithium polymer battery have such an advantage that their energy concentration is substantially high compared to other kinds of batteries. However, they require rigorous charging control with a consideration to deterioration or safety thereof, thus requiring highly-accurate charging control technique.
Therefore, it is preferable to use a charging circuit utilizing a regulator (series regulators, for example) which can highly accurately perform output control for charging a secondary battery.
Considering usability of users, shorter charging time of the secondary battery is favorable. However, charging efficiency of the secondary battery utilizing the contactless power transmission is lower than that of a normal charger (a charger using an AC adaptor) and therefore the secondary battery utilizing the contactless power transmission tends to require longer charging time. Therefore, it is important that power loss in the charging is reduced as much as possible when a mobile terminal is charged by using the contactless power transmission.
According to an inspection of the inventor, it became apparent that power loss occurs in the regulator described above to hinder decrease of the charging time in a case requiring flow of a large amount of charging current such as a case of charging a secondary battery with high exhaustion. Considering the safety of the mobile terminal, it is preferable to minimize the heat generation of the regulator in charging.
Therefore, the inventor studied a technique for reducing power loss and heat generation of the regulator. In the technique, when power feeding capability for the secondary battery is decreased, a bypass path bypassing the regulator is formed and current is supplied to a load (the second battery, for example) which is an object of the power feeding through the bypass path.
As a result of the study, the following thing became apparent. It become apparent that, in power supply through the bypass path bypassing the regulator, overshoot and undershoot might occur in a case where power feeding to the load as the power feeding object is temporarily stopped or in a case where the power feeding under the temporal stop is restarted.
That is, in the case where the power feeding to the load as the power feeding object is temporarily stopped, the current is suddenly stopped. Therefore, a voltage at an output node of the regulator rises sharply, causing overshoot. When the power feeding, which has been temporarily stopped, is restarted, the load rapidly draws great current so as to cause a lack of power supply. Therefore, the voltage at the output node of the regulator is rapidly decreased, causing undershoot. Overshoot and undershoot may cause defects such as a damage of a circuit at the secondary side and require of reset of a control IC (for example, a charging device for controlling charge of the secondary battery) for controlling the power feeding to the load.
Here, in the power feeding to the load as the power feeding object, such controls as the temporary stop of the power feeding and the restart of the power feeding, which has been stopped, may be required when a signal is transmitted from the secondary side to the primary side, for example. That is, if a large amount of current is supplied to the load in the modulation of the load of the power receiving device, the primary side has a difficulty to detect change of a voltage, which occurs by the load modulation, at a coil end of the primary coil. Thus, the primary side has a difficulty to recognize a communication signal produced by the load modulation. Therefore, in a period of the load modulation, the power feeding to the load is temporarily stopped. Accordingly, the primary side can securely detect the communication signal produced by the load modulation. After the completion of the load modulation, the power feeding to the load needs to be restarted.
The communication by the load modulation from the secondary side to the primary side in the power feeding to the load (in a period of a normal power transmission) is required for periodic authentication for foreign object detection (takeover detection), for example. If a foreign object (a thin metal plate, for example) having a large area is inserted between the primary coil and the secondary coil in the normal power transmission from the primary side to the secondary side, the foreign object consumes whole of the power supplied from the primary side. Then the primary side determines that the secondary side device exists, and therefore the normal power transmission is continued. This state is called a “takeover state”. If the takeover state arises, the foreign object generates heat so as to cause accidents such as device damage and burn injury. Therefore, the secondary side device periodically (every second, for example) transmits a signal having a predetermined pattern (“0”, “1”, “0”, for example) by load modulation. When the takeover state does not arise, the primary side can periodically detect a signal having a predetermined pattern and transmitted from the secondary side. On the other hand, when the takeover state arises, the primary side can not detect the signal having the predetermined pattern and transmitted from the secondary side, thereby being able to detect an occurrence of the takeover state. The detection of the takeover state depending on whether the signal periodically transmitted from the secondary side can be received or not is referred to as periodic authentication.
Therefore, in a case where the period during which the bypass path bypassing the regulator for supplying great current to the load is in an ON state overlaps with a period of the periodic authentication, for example, overshoot or undershoot described above may disadvantageously occur.
That is, communication by the load modulation from the secondary side to the primary side is required at the timing of the periodic authentication. At this time, the power feeding to the load is temporarily stopped, so that the current is stopped to increase the voltage at the output node of the regulator, causing overshoot. On the other hand, when the power feeding, which has been temporarily stopped, to the load is restarted after the completion of the load modulation (periodic authentication, the power feeding current can not follow, causing undershoot. Such problems were exposed by the study of the inventor.